The study of sensory-motor processing in the brain of mammals and in that of arthropods shows that similar problems find comparable computational solutions. Topological mapping of sensory data and parallel processing, for example, are two basic principles which we find applied in the visual system of monkeys as well as in the somatosensory system of insects. The study of neural circuit design and information processing in "simpler" nervous systems, with its practical advantages can thus be directly relevant to our understanding of brain function in higher animals. The operations accomplished by complex sensory-motor networks depend on several interrelated factors, such as (i) the intrinsic properties of their component neurones, (ii) the synaptic communication between them, and (iii) the configuration of the circuits they form. Understanding the key of these operations thus requires a precise understanding of the input-output transformation achieved at each of these levels. If we now know in detail, the configuration of the local circuits mediating leg reflexes in insects, we know very little, by contrast, of the intrinsic properties of the various classes of interneurones forming these circuits. In particular, the reason why both spike-mediated and graded communication within and between neurones should exist in parallel remains unclear. Physiological and ultrastructural data both support the idea that the operations which these two types of interneurone perform may be fundamentally different. We thus wish to study the membrane properties of the nonspiking local interneurones together with the transfer at their input and output synapses, to test the hypothesis that some local circuit neurones may operate as several, possibly independent, integrative units. The study of intrinsic cellular properties will, however, also be placed in a functional context. We wish to consider the operation of the local interneurones for local reflex mediation, during a centrally generated motor output. The thoracic local circuits have so far been considered in alert but inactive animals, so that the function and role played by the various interneurones during locomotion for example, are totally unknown. The detailed knowledge that has recently been obtained of the circuits underlying the flow of sensory information, however, now allows us to tackle the specific problem of the central control of local reflex mechanisms.